X-ray crystallographic structures of gramicidin and their relation to the Streptomyces lividans potassium channel structure.

Gramicidin has been used extensively as a model system for structure/function studies of ion channels. Long before crystals of other ion channel proteins were produced, crystals of gramicidin had been prepared, even though it was many years before the first forms of those crystals were solved. There now exist a large number of crystal structures of both uncomplexed and ion-complexed forms of gramicidin crystallized from organic solvents. In all these crystals, the molecules are double helices, although they differ in helical pitch, handedness and side chain orientations, depending on the conditions used for crystallization. Since many of these structures have been discussed in detail in a recent review (Wallace 1998), this chapter concentrates on recently reported structures and how they relate to previously described X-ray and NMR structures. It also discusses how the crystal structure of a K+ complex of gramicidin relates to the recently solved structure of a K+ complex of the potassium channel from Streptomyces lividans and argues that this demonstrates that gramicidin is indeed a good model structure for biological ion channels, despite the presence of D-amino acids in its sequence.